Polyvinyl fluoride (PVF) has been manufactured for many years and has found many uses as a protective film or coating over a variety of substrates because of its excellent weatherability, chemical resistance and mechanical properties. The fluorine atoms in PVF are largely responsible for these properties and, if polymers of higher fluorine content could be made, enhancement of these properties would be achieved. One way to increase polymer fluorine content is to prepare dipolymers in which tetrafluoroethylene (TFE) replaces some of the vinyl fluoride (VF). Such dipolymers were first described by Coffman and Ford, U.S. Pat. No. 2,419,009 (1947) and were produced with a benzoyl peroxide initiator in the presence of vinyl fluoride (VF) monomer and water in a high pressure bomb operated at 122-143 atmospheres pressure. This process produces polymer with nonionic phenyl end groups.
Sianesi and Caporiccio, J. Polymer Sci., Part A-1, 6, (1968) 335, and U.S. Pat. No. 3,513,116 (1970) studied the VF/TFE polymerization process in some detail. In all their examples the initiators used by Sianesi and Caporiccio were organometallic compounds whose decomposition was catalyzed by an oxidizing agent to yield alkyl radicals which initiated polymerization. Alcohols or alcohol/water mixtures were commonly used as the polymerization medium in these processes which afforded copolymers at pressures as low as 1 atmosphere. The initiating alkyl radicals of this process result in nonionic end groups on the polymers.
Stilmar, U.S. Pat. No. 3,531,441 (1970), reported preparation of tri- and tetra-polymers composed of VF, TFE and a wide variety of other vinyl monomers by using organic peroxide initiators in different organic solvents as polymerization media. The initiating radicals from the peroxide initiators formed nonionic end groups on the copolymers. Thus, all known previous work has reported the preparation of VF/TFE copolymers, by using radical generating species that place nonionic end groups on the polymer chains formed. These nonionic end groups are typically alkyl or aryl and, therefore, hydrophobic in nature.
The polymerization processes and attendant copolymers produced by the above prior art methods all have some important disadvantages. The process of Coffman and Ford requires high pressures. This demands expensive, robust high-pressure equipment for manufacture of the copolymer. Additionally, since the initiator is nonionic, the copolymer produced is not wetted by water and forms a lumpy heterogeneous mixture in the reactor. Complete removal of the polymer from the reactor is difficult and can only be accomplished by opening the vessel and manually removing it. This is a slow process with some associated danger and is impractical on all but small scale laboratory equipment.
The processes of Sianesi and Caporiccio require the use of flammable alcohol solvents and toxic organometallics such as tetraethyllead which pose some severe health and environmental risks. The polymerization rates reported for these processes are also low and would force the use of very large scale equipment. These requirements demand appropriate plant design for flammable and toxic materials as well as solvent recovery and refining equipment which serve to drive up equipment and operating costs and complicate the overall process.
Cook, et al. U.S. Pat. No. 3,428,618 (1969) teach the use of cyclic azoamidine free radical initiators in a process for polymerizing fluoroolefins. Although they allude to the preparation of dipolymers and terpolymers of fluoroolefins, examples in the patent are directed to the polymerization of VF homopolymer with cyclic 2,2′-azobis(N,N′-dimethylene-isobutyroamidine) dihydrochloride in preference to acyclic 2,2′-azobis(isobutyroamidine)dihydrochloride. Reference to or examples of other homopolymers or interpolymers, specifically VF/TFE interpolymers, is lacking. There is no recognition of producing a superior interpolymer product containing ionic end groups which product has a small uniform particle size and improved weather resistance, chemical resistance, and stain resistance coupled with improved release properties.